db_file.pm

关于Berkelay数据库的共享源码

    my $filename = "text" ;
    unlink $filename ;

    my @h ;
    tie @h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_RECNO 
        or die "Cannot open file 'text': $!\n" ;

    # Add a few key/value pairs to the file
    $h[0] = "orange" ;
    $h[1] = "blue" ;
    $h[2] = "yellow" ;

    push @h, "green", "black" ;

    my $elements = scalar @h ;
    print "The array contains $elements entries\n" ;

    my $last = pop @h ;
    print "popped $last\n" ;

    unshift @h, "white" ;
    my $first = shift @h ;
    print "shifted $first\n" ;

    # Check for existence of a key
    print "Element 1 Exists with value $h[1]\n" if $h[1] ;

    # use a negative index
    print "The last element is $h[-1]\n" ;
    print "The 2nd last element is $h[-2]\n" ;

    untie @h ;

Here is the output from the script:

    The array contains 5 entries
    popped black
    shifted white
    Element 1 Exists with value blue
    The last element is green
    The 2nd last element is yellow

=head2 Extra RECNO Methods

If you are using a version of Perl earlier than 5.004_57, the tied
array interface is quite limited. In the example script above
C<push>, C<pop>, C<shift>, C<unshift>
or determining the array length will not work with a tied array.

To make the interface more useful for older versions of Perl, a number
of methods are supplied with B<DB_File> to simulate the missing array
operations. All these methods are accessed via the object returned from
the tie call.

Here are the methods:

=over 5

=item B<$X-E<gt>push(list) ;>

Pushes the elements of C<list> to the end of the array.

=item B<$value = $X-E<gt>pop ;>

Removes and returns the last element of the array.

=item B<$X-E<gt>shift>

Removes and returns the first element of the array.

=item B<$X-E<gt>unshift(list) ;>

Pushes the elements of C<list> to the start of the array.

=item B<$X-E<gt>length>

Returns the number of elements in the array.

=item B<$X-E<gt>splice(offset, length, elements);>

Returns a splice of the array.

=back

=head2 Another Example

Here is a more complete example that makes use of some of the methods
described above. It also makes use of the API interface directly (see 
L<THE API INTERFACE>).

    use warnings ;
    use strict ;
    my (@h, $H, $file, $i) ;
    use DB_File ;
    use Fcntl ;

    $file = "text" ;

    unlink $file ;

    $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0666, $DB_RECNO 
        or die "Cannot open file $file: $!\n" ;

    # first create a text file to play with
    $h[0] = "zero" ;
    $h[1] = "one" ;
    $h[2] = "two" ;
    $h[3] = "three" ;
    $h[4] = "four" ;


    # Print the records in order.
    #
    # The length method is needed here because evaluating a tied
    # array in a scalar context does not return the number of
    # elements in the array.  

    print "\nORIGINAL\n" ;
    foreach $i (0 .. $H->length - 1) {
        print "$i: $h[$i]\n" ;
    }

    # use the push & pop methods
    $a = $H->pop ;
    $H->push("last") ;
    print "\nThe last record was [$a]\n" ;

    # and the shift & unshift methods
    $a = $H->shift ;
    $H->unshift("first") ;
    print "The first record was [$a]\n" ;

    # Use the API to add a new record after record 2.
    $i = 2 ;
    $H->put($i, "Newbie", R_IAFTER) ;

    # and a new record before record 1.
    $i = 1 ;
    $H->put($i, "New One", R_IBEFORE) ;

    # delete record 3
    $H->del(3) ;

    # now print the records in reverse order
    print "\nREVERSE\n" ;
    for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
      { print "$i: $h[$i]\n" }

    # same again, but use the API functions instead
    print "\nREVERSE again\n" ;
    my ($s, $k, $v)  = (0, 0, 0) ;
    for ($s = $H->seq($k, $v, R_LAST) ; 
             $s == 0 ; 
             $s = $H->seq($k, $v, R_PREV))
      { print "$k: $v\n" }

    undef $H ;
    untie @h ;

and this is what it outputs:

    ORIGINAL
    0: zero
    1: one
    2: two
    3: three
    4: four

    The last record was [four]
    The first record was [zero]

    REVERSE
    5: last
    4: three
    3: Newbie
    2: one
    1: New One
    0: first

    REVERSE again
    5: last
    4: three
    3: Newbie
    2: one
    1: New One
    0: first

Notes:

=over 5

=item 1.

Rather than iterating through the array, C<@h> like this:

    foreach $i (@h)

it is necessary to use either this:

    foreach $i (0 .. $H->length - 1) 

or this:

    for ($a = $H->get($k, $v, R_FIRST) ;
         $a == 0 ;
         $a = $H->get($k, $v, R_NEXT) )

=item 2.

Notice that both times the C<put> method was used the record index was
specified using a variable, C<$i>, rather than the literal value
itself. This is because C<put> will return the record number of the
inserted line via that parameter.

=back

=head1 THE API INTERFACE

As well as accessing Berkeley DB using a tied hash or array, it is also
possible to make direct use of most of the API functions defined in the
Berkeley DB documentation.

To do this you need to store a copy of the object returned from the tie.

	$db = tie %hash, "DB_File", "filename" ;

Once you have done that, you can access the Berkeley DB API functions
as B<DB_File> methods directly like this:

	$db->put($key, $value, R_NOOVERWRITE) ;

B<Important:> If you have saved a copy of the object returned from
C<tie>, the underlying database file will I<not> be closed until both
the tied variable is untied and all copies of the saved object are
destroyed. 

    use DB_File ;
    $db = tie %hash, "DB_File", "filename" 
        or die "Cannot tie filename: $!" ;
    ...
    undef $db ;
    untie %hash ;

See L<The untie() Gotcha> for more details.

All the functions defined in L<dbopen> are available except for
close() and dbopen() itself. The B<DB_File> method interface to the
supported functions have been implemented to mirror the way Berkeley DB
works whenever possible. In particular note that:

=over 5

=item *

The methods return a status value. All return 0 on success.
All return -1 to signify an error and set C<$!> to the exact
error code. The return code 1 generally (but not always) means that the
key specified did not exist in the database.

Other return codes are defined. See below and in the Berkeley DB
documentation for details. The Berkeley DB documentation should be used
as the definitive source.

=item *

Whenever a Berkeley DB function returns data via one of its parameters,
the equivalent B<DB_File> method does exactly the same.

=item *

If you are careful, it is possible to mix API calls with the tied
hash/array interface in the same piece of code. Although only a few of
the methods used to implement the tied interface currently make use of
the cursor, you should always assume that the cursor has been changed
any time the tied hash/array interface is used. As an example, this
code will probably not do what you expect:

    $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
        or die "Cannot tie $filename: $!" ;

    # Get the first key/value pair and set  the cursor
    $X->seq($key, $value, R_FIRST) ;

    # this line will modify the cursor
    $count = scalar keys %x ; 

    # Get the second key/value pair.
    # oops, it didn't, it got the last key/value pair!
    $X->seq($key, $value, R_NEXT) ;

The code above can be rearranged to get around the problem, like this:

    $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
        or die "Cannot tie $filename: $!" ;

    # this line will modify the cursor
    $count = scalar keys %x ; 

    # Get the first key/value pair and set  the cursor
    $X->seq($key, $value, R_FIRST) ;

    # Get the second key/value pair.
    # worked this time.
    $X->seq($key, $value, R_NEXT) ;

=back

All the constants defined in L<dbopen> for use in the flags parameters
in the methods defined below are also available. Refer to the Berkeley
DB documentation for the precise meaning of the flags values.

Below is a list of the methods available.

=over 5

=item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>

Given a key (C<$key>) this method reads the value associated with it
from the database. The value read from the database is returned in the
C<$value> parameter.

If the key does not exist the method returns 1.

No flags are currently defined for this method.

=item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>

Stores the key/value pair in the database.

If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
will have the record number of the inserted key/value pair set.

Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
R_SETCURSOR.

=item B<$status = $X-E<gt>del($key [, $flags]) ;>

Removes all key/value pairs with key C<$key> from the database.

A return code of 1 means that the requested key was not in the
database.

R_CURSOR is the only valid flag at present.

=item B<$status = $X-E<gt>fd ;>

Returns the file descriptor for the underlying database.

See L<Locking: The Trouble with fd> for an explanation for why you should
not use C<fd> to lock your database.

=item B<$status = $X-E<gt>seq($key, $value, $flags) ;>

This interface allows sequential retrieval from the database. See
L<dbopen> for full details.

Both the C<$key> and C<$value> parameters will be set to the key/value
pair read from the database.

The flags parameter is mandatory. The valid flag values are R_CURSOR,
R_FIRST, R_LAST, R_NEXT and R_PREV.

=item B<$status = $X-E<gt>sync([$flags]) ;>

Flushes any cached buffers to disk.

R_RECNOSYNC is the only valid flag at present.

=back

=head1 DBM FILTERS

A DBM Filter is a piece of code that is be used when you I<always>
want to make the same transformation to all keys and/or values in a
DBM database.

There are four methods associated with DBM Filters. All work identically,
and each is used to install (or uninstall) a single DBM Filter. Each
expects a single parameter, namely a reference to a sub. The only
difference between them is the place that the filter is installed.

To summarise:

=over 5

=item B<filter_store_key>

If a filter has been installed with this method, it will be invoked
every time you write a key to a DBM database.

=item B<filter_store_value>

If a filter has been installed with this method, it will be invoked
every time you write a value to a DBM database.


=item B<filter_fetch_key>

If a filter has been installed with this method, it will be invoked
every time you read a key from a DBM database.

=item B<filter_fetch_value>

If a filter has been installed with this method, it will be invoked
every time you read a value from a DBM database.

=back

You can use any combination of the methods, from none, to all four.

All filter methods return the existing filter, if present, or C<undef>
in not.

To delete a filter pass C<undef> to it.

=head2 The Filter

When each filter is called by Perl, a local copy of C<$_> will contain
the key or value to be filtered. Filtering is achieved by modifying
the contents of C<$_>. The return code from the filter is ignored.

=head2 An Example -- the NULL termination problem.

Consider the following scenario. You have a DBM database
that you need to share with a third-party C application. The C application
assumes that I<all> keys and values are NULL terminated. Unfortunately
when Perl writes to DBM databases it doesn't use NULL termination, so
your Perl application will have to manage NULL termination itself. When
you write to the database you will have to use something like this:

    $hash{"$key\0"} = "$value\0" ;

Similarly the NULL needs to be taken into account when you are considering
the length of existing keys/values.

It would be much better if you could ignore the NULL terminations issue
in the main application code and have a mechanism that automatically
added the terminating NULL to all keys and values whenever you write to
the database and have them removed when you read from the database. As I'm
sure you have already guessed, this is a problem that DBM Filters can
fix very easily.

    use warnings ;
    use strict ;
    use DB_File ;

    my %hash ;
    my $filename = "filt" ;
    unlink $filename ;

    my $db = tie %hash, 'DB_File', $filename, O_CREAT|O_RDWR, 0666, $DB_HASH 
      or die "Cannot open $filename: $!\n" ;

    # Install DBM Filters
    $db->filter_fetch_key  ( sub { s/\0$//    } ) ;
    $db->filter_store_key  ( sub { $_ .= "\0" } ) ;
    $db->filter_fetch_value( sub { s/\0$//    } ) ;
    $db->filter_store_value( sub { $_ .= "\0" } ) ;